Complexation color development-Infrared spectroscopy (CCDIR) for linear detection of high-concentration Ca2+ in mineral processing circulating water and optimization of green processes

Complexation color development-infrared spectroscopy (CCDIR) measures Ca 2+ directly. • CCDIR remains linear at 10 ∼ 150 mg/L Ca 2+ , unlike visible spectrophotometry. • CCDIR achieves a 0.354 mg/L detection limit and 96 ∼ 104% recoveries. • The infrared response of CCDIR method remains measurable from pH 3 to 12. • This method exhibits strong anti-interference ability to Na⁺ and K⁺. Conventional complexation color development-visible spectrophotometry (CCDVS) is limited for direct determination of high-concentration Ca 2+ in mineral-processing circulating water because it becomes nonlinear at high concentrations. To address this limitation, this study integrates complexation color development with infrared spectroscopy and proposes a novel detection approach, complexation color development-infrared spectroscopy (CCDIR). At pH = 11, CCDIR was linear over 0 ∼ 10 mg/L (T = 0.1328C + 2.3903, R 2  = 0.999) and 10 ∼ 150 mg/L (T = 0.0246C + 3.5370, R 2  = 0.999), whereas CCDVS was linear only at 0–10 mg/L (A = -0.1265C-0.0579, R 2  = 0.993) and presented a nonlinear model at 10 ∼ 150 mg /L. The limits of detection were 0.354 and 0.372 mg/L for CCDIR and CCDVS, respectively. Across the CCDIR samples studied, relative error and relative standard deviation remained below 4%, corresponding to recoveries of 96 ∼ 104%. CCDVS relies on a specific pH (pH = 11), and pH fluctuations significantly reduce its accuracy. In contrast, CCDIR shows a wider pH adaptation window (3 ∼ 12), offering greater environmental adaptability and stability. A comparison of the calibration slopes revealed that CCDIR offered a more stable and predictable response over its extended linear range compared to the limited linear range and subsequent nonlinear behavior of CCDVS. At 150 mg/L, 20 mg/L Na + and K + changed the visible signal by only 0.36% and 0.32%, respectively, whereas Mg 2+ produced measurable interference. CCDIR technology provides technical support for accurate determination of high-concentration Ca 2+ in mineral processing wastewater, as well as data support for rapid detection of metal ions in coal washing and processing processes.